Network units such as switches which include a multiplicity of ports for connection to other units or users are commonly made with a fixed number of ports in order to achieve efficiency of manufacture. It is known to ‘stack’ or ‘cascade’ such units by means of interconnection and a common control or management system to constitute a single switching or routing entity from a multiplicity of individual units. A particular example is disclosed in U.S. patent application Ser. No. 10/093,506 of Weyman et al, published as US2003-0147412-A1.
As is known, routers, as distinct from bridges, are required amongst other things to route packets between different networks or sub-nets. For this purpose they are required to perform a substantial processing on packets, such as a change in relevant MAC addresses in accordance with a selected route to a destination defined by a network address. Moreover, a router preferably is capable of performing a routing protocol, which normally comprises an advertising function, by means of which a router informs other routers by way of routing information packets of the routers and networks it can reach, and a listening function by responding to such packets to build up routing tables, which are used to generate forwarding data such as MAC addresses and port numbers indexed against the network addresses of packets.
The aforementioned application describes a cascaded stack of routing units each of which is capable of routing packets that it receives at its external ports. The routing units can conduct an election for a lead router that lends network and media access control addresses to the router interfaces of the other units in the stack. The stack is organised so that only the lead router performs a full routing protocol whereas the subordinate units have selected functions within the routing protocol suppressed but have a full routing capability.
An important aspect of stacked routers is the provision of resilience, that is to say the provision of a facility that enables the stack or at least a remainder of the stack to continue in appropriate operation notwithstanding the failure of a connection between units in a stack or the failure of one of the units in a stack. Resilience normally requires the provision of at least one closed path or multiplicity of closed paths in the stack or cascade of networks. A simple example is the connection of the ‘top’ unit of a stack to the ‘bottom’ unit of the stack so as to permit connectivity between the units of the stack notwithstanding the failure of any one of them.
Alternatively, connectivity may be maintained with the aid of neighboring switching units that are connected to more than one unit in the stack.
An aspect of resilience, particularly if it involves units not in the stack is the enabling of new elections where a set of functional units are separated from another unit or units in the stack. Bearing in mind that temporarily disrupted stacks may be reconnected, a further aspect of this is the avoidance of two elected masters in the same stack or remainder of a stack. Co-pending U.S. patent application Ser. No. 10/195,503 of Weyman et al. (published as 2003-0169748-A) describes stackable router units wherein a lead or master unit can transit to a master routing state and in that state broadcast its address as an advertisement to other units on the network. When a unit receives the advertisement it can transit to a master bridging state and its routing facility is disabled. The purpose of this facility is to enable a single unit in a segment of a stack to become a master unit and also to enable negotiation between the master units of respective segments to determine which of them will be the single lead or master unit for the reconnected stack and would therefore have control of the routing interfaces in the stack according to the abovementioned applications.
Further details of resilient stacked routers are also disclosed in the aforementioned patent applications, the disclosures of which are incorporated herein by reference.
It is mentioned above that one of the functions performed by a router is the executing of a routing protocol to determine a route to a network destination. One important protocol for this purpose is the OSPF protocol. The OSPF protocol is fully described in its present form in for example RFC-2328 OSPF Version 2 (Ascend Communications Inc, April 1998). It is presumed in the following that the reader has a knowledge of that protocol, which is incorporated by reference herein. Another routing protocol is known as RIP (Routing Information Protocol).